This invention relates to a method of improving silicone resin coating compositions. More particularly, it relates to a method of upgrading silicone resin coating compositions by ultrafiltration.
Silicone resin coating compositions are presently being utilized to protect the surface of wood, metals, and synthetic polymers. These transparent coatings provide excellent mar and scratch resistance to acrylic and polycarbonate sheets used as glazing. An example of a widely used transparent polycarbonate glazing on which these coating are useful is Lexan.RTM. polycarbonate resin sold by General Electric Company.
The silicone resin coating formulations typically comprise an aqueous dispersion of colloidal silica or silica gel and hydrolizable silanes, such as the trifunctional silanes of the formula, R'Si(OR).sub.3 and a solvent medium such as alcohol and water.
Degradation of these silicone coating compositions from prolonged exposure to moisture, humidity and ultra violet light during outdoor use, has presented certain problems. Yellowing and hazing along with delamination of these protective coatings is often observed. Attempts to enhance the longevity of such silicone coatings have included modification of the solvent carrier, as disclosed by Anthony in Ser. No. 373,361, filed Dec. 1, 1982 and modifications of the composition. These modifications include the introduction of additives to the coating as disclosed by Frye in U.S. Pat. No. 4,277,287 and altering the pH of the composition as disclosed by Vaughn, Jr. in U.S. Pat. No. 4,368,235. The above references are assigned to the same assignee as the present invention.
While the procedures mentioned above have provided acceptable coating formulations, there still remains room for improvement. For example, the method comprising this invention upgrades the coatings contained in the processes identified above in addition to providing coatings having improved resistance to ultra violet light, heat, moisture, stress and strain.